1. Field of the Inventions
The present inventions relate generally to electrophysiological devices and, more particularly, to the indifferent electrodes that are used in conjunction with electrophysiological devices.
2. Description of the Related Art
There are many instances where diagnostic and therapeutic elements must be inserted into the body. One instance involves the treatment of cardiac conditions such as atrial fibrillation and atrial flutter which lead to an unpleasant, irregular heart beat, called arrhythmia.
Normal sinus rhythm of the heart begins with the sinoatrial node (or “SA node”) generating an electrical impulse. The impulse usually propagates uniformly across the right and left atria and the atrial septum to the atrioventricular node (or “AV node”). This propagation causes the atria to contract in an organized way to transport blood from the atria to the ventricles, and to provide timed stimulation of the ventricles. The AV node regulates the propagation delay to the atrioventricular bundle (or “HIS” bundle). This coordination of the electrical activity of the heart causes atrial systole during ventricular diastole. This, in turn, improves the mechanical function of the heart. Atrial fibrillation occurs when anatomical obstacles in the heart disrupt the normally uniform propagation of electrical impulses in the atria. These anatomical obstacles (called “conduction blocks”) can cause the electrical impulse to degenerate into several circular wavelets that circulate about the obstacles. These wavelets, called “reentry circuits,” disrupt the normally uniform activation of the left and right atria.
Because of a loss of atrioventricular synchrony, the people who suffer from atrial fibrillation and flutter also suffer the consequences of impaired hemodynamics and loss of cardiac efficiency. They are also at greater risk of stroke and other thromboembolic complications because of loss of effective contraction and atrial stasis.
Although pharmacological treatment is available for atrial fibrillation and flutter, the treatment is far from perfect. For example, certain antiarrhythmic drugs, like quinidine, amiodarone, and procainamide, can reduce both the incidence and the duration of atrial fibrillation episodes. Yet, these drugs often fail to maintain sinus rhythm in the patient. Cardioactive drugs, like digitalis, Beta blockers, and calcium channel blockers, can also be given to control the ventricular response. However, many people are intolerant to such drugs. Anticoagulant therapy also combats thromboembolic complications, but does not eliminate them. Unfortunately, pharmacological remedies often do not remedy the subjective symptoms associated with an irregular heartbeat. They also do not restore cardiac hemodynamics to normal and remove the risk of thromboembolism.
Many believe that the only way to really treat all three detrimental results of atrial fibrillation and flutter is to actively interrupt all of the potential pathways for atrial reentry circuits.
One surgical method of treating atrial fibrillation by interrupting pathways for reentry circuits is the so-called “maze procedure” which relies on a prescribed pattern of incisions to anatomically create a convoluted path, or maze, for electrical propagation within the left and right atria. The incisions direct the electrical impulse from the SA node along a specified route through all regions of both atria, causing uniform contraction required for normal atrial transport function. The incisions finally direct the impulse to the AV node to activate the ventricles, restoring normal atrioventricular synchrony. The incisions are also carefully placed to interrupt the conduction routes of the most common reentry circuits. The maze procedure has been found very effective in curing atrial fibrillation. However, the maze procedure is technically difficult to do. It also requires open heart surgery and is very expensive. Thus, despite its considerable clinical success, only a few maze procedures are done each year.
Maze-like procedures have also been developed utilizing catheters which can form lesions on the endocardium to effectively create a maze for electrical conduction in a predetermined path. Exemplary catheters are disclosed in commonly assigned U.S. Pat. No. 5,582,609. Typically, the lesions are formed by ablating tissue with one or more electrodes carried by the catheter. Electromagnetic radio frequency (“RF”) energy applied by the electrodes heats, and eventually kills (i.e. “ablates”), the tissue to form a lesion. During the ablation of soft tissue (i.e. tissue other than blood, bone and connective tissue), tissue coagulation occurs and it is the coagulation that kills the tissue. Thus, references to the ablation of soft tissue are necessarily references to soft tissue coagulation. “Tissue coagulation” is the process of cross-linking proteins in tissue to cause the tissue to jell. In soft tissue, it is the fluid within the tissue cell membranes that jells to kill the cells, thereby killing the tissue.
Catheters used to create lesions (the lesions being 3 to 15 cm in length) typically include a relatively long and relatively flexible body portion that has a plurality electrodes supported or near its distal end. The portion of the catheter body portion that is inserted into the patient is typically from 23 to 55 inches in length and there may be another 8 to 15 inches, including a handle, outside the patient. The proximal end of the catheter body is connected to the handle which includes steering controls. The length and flexibility of the catheter body allow the catheter to be inserted into a main vein or artery (typically the femoral artery), directed into the interior of the heart, and then manipulated such that the electrode contacts the tissue that is to be ablated. Fluoroscopic imaging is used to provide the physician with a visual indication of the location of the catheter.
Although catheter-based soft tissue coagulation has proven to be a significant advance in the medical arts generally and in the treatment of cardiac conditions in particular, it is not appropriate in every situation. Physicians may, for example, desire to perform a maze procedure as a supplemental procedure during an open heart surgical procedure such as a mitral valve replacement. Physicians may also desire to form lesions on the epicardial surface. Surgical probes which include a relatively short shaft that supports a plurality of electrodes have been introduced in recent years to facilitate the formation of lesions in these situations. Exemplary surgical probes are disclosed in commonly assigned U.S. Pat. No. 6,142,994, which is entitled “Surgical Method And Apparatus For Introducing Diagnostic And Therapeutic Elements Within The Body,” which is incorporated here by reference.
Soft tissue coagulation that is performed using electrodes to transmit energy to tissue, whether catheter-based or surgical probe-based, may be performed in both bi-polar and uni-polar modes. Both modes require one or more indifferent return electrodes. In the uni-polar mode, energy emitted by the electrodes supported on the catheter or surgical probe is returned through one or more indifferent patch electrodes that are externally attached to the skin of the patient. Bi-polar devices, on the other hand, typically include a number of bi-polar electrode pairs. Both electrodes in each pair are supported by the catheter or surgical probe and energy emitted by one electrode in a particular pair is returned by way of the other electrode in that pair.
The uni-polar mode has proven to be superior to the bi-polar mode because the uni-polar mode allows for individual electrode control, while the bi-polar mode only allows electrode pairs to be controlled. Nevertheless, the inventor herein has determined that conventional uni-polar soft tissue coagulation techniques can be problematic because some patients have delicate skin and/or skin infections that preclude the attachment of an indifferent patch electrode to their skin. Poor indifferent electrode/skin contact can also be a problem, as can local burning. The inventor herein has also determined that it would be desirable to improve the likelihood that soft tissue coagulation procedures will result in transmural lesions, which is not always the case when conventional techniques are employed.